HD 128717 b
Astrophysical Synthesis & Habitability Characterization
HD 128717 b is a confirmed gas giant exoplanet orbiting the star HD 128717, located approximately 240.2 light-years (73.64 parsecs) from the Solar System. First announced in 2025 by European Space Agency (ESA) Gaia Satellite using the Astrometry detection technique, this world measures 12.1 Earth radii (1.08 Jupiter radii) and has a measured mass of 6293.002 Earth masses (19.8 Jupiter masses). Combining its mass and radius yields a mean bulk density of 19.59 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 42.98 g⊕ (421.49 m/s²), and an escape velocity of 255.1 km/s.
In its orbital architecture around HD 128717, HD 128717 b completes one full revolution every 3420 Earth days (9.36Julian years) at a semi-major axis of 4.85 AU (corresponding to the middle-system regime between Mars at 1.52 AU and Jupiter at 5.20 AU). Its mean orbital velocity is approximately 15.4 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.85, swinging from 0.7275 AU at periapsis out to 8.9725 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
❄️ Thermal Regime (Cryogenic Zone): With an equilibrium temperature of 152 K (-121°C / -186°F) and an incident flux of 0.09 S⊕ (~121 W/m²), surface water and volatile species on HD 128717 b would remain locked in cryogenic ice phases unless warmed by subsurface tidal or radiogenic heating.
The host star HD 128717 is classified as a stellar host with an effective photospheric temperature of 6,210 K, a radius of 1.248 R☉, a mass of 1.212 M☉, an estimated bolometric luminosity of 2.078 L☉. Stellar evolution modeling indicates an estimated stellar age of 1.4 Gyr and a metallicity of [Fe/H] = 0.16 dex (metal-rich relative to the Sun). From Earth, HD 128717 shines at apparent V-band magnitude 8.32 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: European Space Agency (ESA) Gaia Satellite · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 128717 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 6,210 K |
| Stellar Radius | 1.2480 R☉ |
| Stellar Mass | 1.2120 M☉ |
| Bolometric Luminosity | 2.078 L☉ |
| Metallicity [Fe/H] | 0.160 dex |
| Stellar Age | 1.400 Gyr |
| Apparent Magnitude | V: 8.320 · G: 8.189 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 4.850000 AU |
| Orbital Period | 3420 days |
| Mean Orbital Velocity | 15.4 km/s |
| Orbital Eccentricity (e) | 0.85000 |
| Orbital Inclination (i) | 130.3000° |
| Mean Bulk Density | 19.59 g/cm³ (255.1 km/s vesc) |
| Insolation Flux | 0.0892 S⊕ (~121 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 240.200 LY (73.636 pc) |